java - 一个简单多线程资源共享问题，不明白为什么会出现奇数结果

代码为下面：

1. 为什么运行会出现奇数结果？ 《java编程思想》的里面的解释没有看懂，能够请大神解释一下其中原理。

import java.util.concurrent.*;

public class EvenGenerator extends IntGenerator {
  private int currentEvenValue = 0;
  public int next() {
    ++currentEvenValue; // Danger point here!
    ++currentEvenValue;
    return currentEvenValue;
  }
  public static void main(String[] args) {
    EvenChecker.test(new EvenGenerator() ,10);
  }
} 
public class EvenChecker implements Runnable {
  private IntGenerator generator;
  private final int id;
  public EvenChecker(IntGenerator g, int ident) {
    generator = g;
    id = ident;
  }
  public void run() {
    while(!generator.isCanceled()) {
      int val = generator.next();
    //   System . out . println ( val ) ;         
      if(val % 2 != 0) {
        System.out.println(val + " not even!");
        generator.cancel(); // Cancels all EvenCheckers
      }
    }
  }
  // Test any type of IntGenerator:
  public static void test(IntGenerator gp, int count) {
    System.out.println("Press Control-C to exit");
    ExecutorService exec = Executors.newCachedThreadPool();
    for(int i = 0; i < count; i++)
      exec.execute(new EvenChecker(gp, i));
    exec.shutdown();
  }
 
  }
}